Catalytic domain of the Protein Serine/Threonine Kinase, Class IIIA myosin. Serine/threonine kinases (STKs), class IIIA myosin subfamily, catalytic (c) domain. STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. The class III myosin subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. Class III myosins are motor proteins containing an N-terminal kinase catalytic domain and a C-terminal actin-binding domain. Class III myosins may play an important role in maintaining the structural integrity of photoreceptor cell microvilli. In photoreceptor cells, they may also function as cargo carriers during light-dependent translocation of proteins such as transducin and arrestin. Class IIIA myosin is highly expressed in retina and in inner ear hair cells. It is localized to the distal ends of actin-bundled structures. Mutations in human myosin IIIA are responsible for progressive nonsyndromic hearing loss. Human myosin IIIA possesses ATPase and kinase activities, and the ability to move actin filaments in a motility assay. It may function as a cellular transporter capable of moving along actin bundles in sensory cells.